1. Field of the Invention
The present invention relates to a hydraulic circuit, a precedence valve block and an operation valve block assembly, and more particularly, to: a hydraulic circuit for distributing pressure oil discharged from a single pump to a high-pressure side circuit in which a high-pressure specified operation valve is provided and to a low-pressure side circuit in which a low-pressure specified operation valve is provided; a precedence valve block having a port structure for distributing the pressure oil to the high-pressure side circuit and the low-pressure side circuit; and an operation valve block assembly in which a plurality of operation valve blocks each having an operation valve are assembled.
2. Description of the Related Art
In a vehicle including a construction machine or a working machine such as a bulldozer and the like, valves such as a tilt valve for driving a tilt cylinder or a steering valve for driving a steering motor and a variable capacitance pump connected to a pipe which supplies a pressure oil are mounted. For example, a hydraulic circuit having the valve, an oil pipe and the like is assembled into a block as one unit. In a conventional hydraulic circuit, as shown in FIG. 9 for example, a precedence valve 5 for distributing a pressure oil to a steering circuit 3 and a working machine circuit 4 is disposed in a discharge pipe 2 of a single variable capacitance pump 1.
The precedence valve 5 includes an input port 6 which communicates with the pump 1, two output ports 7 and 8 connected to the steering circuit 3 and the working machine circuit 4, and first and second pilot pressure receiving portions 9 and 10. A pressure oil discharged from the pump 1 passes through the precedence valve 5 and is distributed to the steering circuit 3 and the working machine circuit 4. The steering valve 12 is connected to the output port 7 of the precedence valve 5. Four closed center-type operation valves 13, e.g., lift valves or the like are connected to the output port 8 through pressure compensation valves 14, respectively.
Various kinds of working machine actuators (not shown) such as a lift cylinder, a tilt cylinder and the like are connected to the working machine operation valve 13 through a connection circuit 15, respectively. The discharge pipe 2 of the pump 1 includes a main relief valve 17 for limiting a maximum pressure of the hydraulic circuit. The main relief valve 17 is connected to a drain circuit 19 through a back pressure valve 18. The discharge pipe 2 of the pump 1 includes an unload valve 16. The unload valve 16 unloads when a pressure difference between a pump discharging pressure and a maximum load pressure of the load pressure applied to the actuator exceeds a set value. The unloaded returning pressure oil is discharged to the drain circuit 19.
The pump 1 includes a load sensing valve 11. The load sensing valve 11 is operated such as to keep the pressure difference between discharging pressure of the pump 1 and the maximum load pressure of the load pressure applied to the actuator at a constant value. More specifically, an output pressure of the load sensing valve 11 is introduced into a pressure receiving portion of a servo piston which is a member for controlling a swash plate angle of the pump 1. The servo piston is moved by the discharge pressure of the pump 1 and the output pressure of the load sensing valve 11. The pump swash plate angle is varied by the movement of the servo piston. In the conventional hydraulic circuit, when a load is applied to the steering motor, the flow rate in the output port 7 connected to the steering circuit 4 is increased by the precedence valve 5. At that time, the flow rate to the output port 8 connected to the working machine circuit is reduced.
The hydraulic circuit shown in FIG. 9 is a hydraulic circuit for a bulldozer. This hydraulic circuit can be applied to a hydraulic shovel. In this case, the steering circuit is a running motor circuit having two operation valves. Further, in the working machine circuit, if the number of operation valves is increased or reduced in accordance with the number of the actuators, it is possible to form the precedence circuit using the precedence valve 5 in quite the same manner. Generally, the above-described pressure compensation valve is provided to each of a plurality of flow rate controlling valves and has a function to equalize pressure differences at input side and output side respectively of each of the flow rate controlling valves. With this function, it is possible to output flow rate in accordance with an opening degree of the flow rate controlling valve irrespective of load pressures which are different from one actuator to another actuator. This function is known, and many kinds of connecting method of the operation compensation valve and structure thereof are known. Further, it is common that the operation valve, the pressure compensation valve and the like are connected to each other to form blocks or are assembled into a single block.
In the above-described hydraulic circuit, a maximum pressure of the circuit is set in accordance with an actuator whose load is expected to be the greatest. In the case of the bulldozer, the maximum pressure thereof is set such as to secure a pressure and a flow rate necessary for the steering motor. In the case of a hydraulic shovel also, a relatively great load pressure is applied to a running motor.
A block having an operation valve and the like is produced in accordance with the maximum pressure and the maximum flow rate. In such a case, a block having an operation valve and the like is increased in size and weight. However, checking each and every actuators, there exists some actuators which need not be produced in accordance with the maximum pressure and the maximum flow rate.
If a vehicle such as a construction machine is relatively large, it is easy to mount therein a large operation valve block which realizes the conventional hydraulic circuit. However, in the case of a relatively small vehicle, a mounting space for the operation valve block may be limited. That is, there is a problem that an operation valve block which was produced in accordance with the maximum pressure and the maximum flow rate can not be mounted.
Generally, in the case of a vehicle such as a relatively small construction machine, a steering motor and a running motor require high pressure and large flow rate, and a working machine circuit only requires relatively low pressure and small flow rate. As the vehicle is smaller, a difference between required high pressure and low pressure, and a difference of required flow rate are more remarkable.
However, it is not possible to employ an operation valve specified for low pressure and small flow rate only for the working machine circuit using the conventional hydraulic circuit to reduce the vehicle in size and weight. The conventional hydraulic circuit is a circuit in which input pressures to the steering valve 12 and pressure compensation valves 14 connected to the working machine operation valves 13 are the same. Therefore, even when only the working machine circuit is specified for low-pressure, the pump pressure is increased in accordance with a load pressure of the steering valve 12, and the pressure adversely exceeds the specified pressure of the low-pressure specified operation valve. Further, the pressure oil adversely flows into low-pressure side with high priority, and there is a problem that a vehicle can not be produced with this design.